Method for characterizing the timbre of a sound signal in accordance with at least a descriptor

ABSTRACT

The invention concerns a method for characterizing in accordance with at least a descriptor, the timbre of a time-variable sound signal (s(t) for a duration D, which consists in defining said descriptor by the harmonic spectral scope of the signal.

[0001] The invention relates to a process for characterisation of the timbre of a sound signal, according to at least one descriptor.

[0002] The domain of the invention is characterisation of the timbre of a sound signal varying as a function of time.

[0003] The timbre of a sound signal is characterised intuitively by all perceptive properties excluding the tone pitch, the perceived intensity and the subjective duration of the sound signal.

[0004] Characteristics vary as a function of the various categories of sound signals. For example, a distinction is made between harmonic sound signals such as sounds produced by a violin, a flute, etc., and percussive sound signals such as those produced by a drum, etc. Obviously, there are other categories.

[0005] Timbre measurements were made for harmonic and percussive sound signal categories: each of these measurement assemblies forms either a harmonic or percussive timbre space.

[0006] An attempt is made to model the timbre of a sound signal s(t), or more precisely to model its characteristics also called descriptors, for example so as to be able to recognise or locate the timbre of an unknown signal, among known timbres in a sound database. Models of these characteristics are usually expressed as a function of spectral and time envelopes of the sound signal s(t) and of their variation.

[0007] The sound signal s(t) and the time envelope ET(t) are illustrated in FIG. 1; the spectral envelope ES(f) is illustrated in FIG. 3; it is usually obtained following a first step consisting of analysing the signal according to a sliding time window, an example of which is shown in FIG. 2, followed by a second step consisting of calculating the Fast Fourier Transform of the signal resulting from the previous step.

[0008] Example models of characteristics, and calculations of the distance between the timbres of the two sound signals in the same timbre space as a function of these characteristics, are suggested in the publication “validation of a multidimensional distance model for perceptual dissimilarities among musical timbres” N. Misdariis et al., Proceedings of the 16^(th) International Congress on Acoustics and 135^(th) Meeting Acoustical Society of America, Seattle, Wash., 20-26 Jun. 1998.

[0009] These characteristics include the following, some of which are presented in the publication mentioned:

[0010] the logarithmic attack time (lat or LT) defined as being the logarithm of the difference between the time t0 at which the signal starts and the time t1 at which the signal stabilises as in the case of harmonic sound signals, or reaches its maximum as in the case of percussive sound signals; lat=log₁₀(t1−t0); these times t0 and t1 are shown in FIG. 1; in the publication mentioned, t0 is the time at which the signal amplitude reaches 2% of the maximum amplitude;

[0011] the harmonic spectral centroid (hsc) or SC defined as being the average of the instantaneous spectral centroid over the duration of the signal, in other words considered in a sliding analysis window;

[0012] the instantaneous spectral centroid itself is defined by the weighted average of harmonic peaks of the spectrum of the signal represented in FIG. 3 and in a way corresponds to the equilibrium point of all harmonic peaks.

[0013] One simple method among the methods of obtaining harmonic peaks of a signal consists firstly of extracting the fundamental frequency f0 of the sound signal s(t), and then secondly detecting harmonic peaks located around multiples of the fundamental frequency f0 as illustrated in FIG. 3. For example, the local fundamental frequency may be obtained by calculating the normalised self-correlation function of the local signal s(t); the local fundamental frequency f0 then corresponds to the inverse of the time T0 of the first maximum of this function;

[0014] the harmonic spectral deviation (hsd) representative of the spectral irregularity defined as being the average of the instantaneous harmonic spectral deviation considered in a sliding analysis window, over the duration of the signal; the instantaneous harmonic spectral deviation itself is defined as being the spectral deviation of amplitude peaks (in a logarithmic scale) of the spectrum with respect to the spectral envelope. An example of an instantaneous harmonic spectral deviation (ihsd) corresponding to the sound signal of a clarinet is illustrated in FIG. 4;

[0015] the harmonic spectral variation (hsv) representative of the spectral flux, defined as being the average of the instantaneous harmonic spectral variation over the duration of the signal, considered in an analysis window; the instantaneous harmonic spectral variation itself is defined as being the 1's complement of the normalised correlation between the amplitude of harmonics in two adjacent windows.

[0016] Therefore the purpose of this invention is to define new characteristics or descriptors so that when combined with known descriptors, they are at best applicable to different timbre spaces and are used to make optimum calculations of the distance between two sound signals within the same timbre space.

[0017] The purpose of the invention is a process for characterisation of the timbre of a sound signal s(t) varying as a function of time for a duration D according to at least one descriptor, characterized mainly in that it consists of defining the said descriptor by the harmonic spectral spread (hss) of the signal.

[0018] According to one characteristic of the invention, one of the descriptors being the harmonic spectral centroid (hsc), the harmonic spectral spread of the signal is calculated according to the following steps:

[0019] a) memorise the signal s(t),

[0020] b) extract its fundamental frequency f0,

[0021] c) calculate and memorise harmonics of the signal s(t) truncated within a time window h(t) with a duration less than or equal to D, as a function of the frequency using a fast Fourier transform system, making the said time window h(t) slide over the duration D of the signal s(t),

[0022] d) for each time window h(t), calculate the harmonic spectral spread of the truncated signal hss(s(t).h(t)) using the following formula: ${{hss}\left( {s.h} \right)} = {\frac{1}{{hsc}\left( {s.h} \right)}\sqrt{\frac{\sum\limits_{nbh}\quad {{A^{2}\left( {{s.h},{harm}} \right)}\left\lbrack {{f\left( {{s.h},{harm}} \right)} - {{hsc}\left( {s.h} \right)}} \right\rbrack}^{2}}{\sum\limits_{nbh}\quad {A^{2}\left( {{s.h},{harm}} \right)}}}}$

[0023] where A(s.h, harm) is the amplitude of harmonic peak number harm of the spectrum of the truncated signal s.h,

[0024] f(s.h, harm) is the frequency of harmonic number harm of the spectrum of the truncated signal,

[0025] nbh is the number of harmonics in the spectrum of the truncated signal s.h,

[0026] hsc(s.h) is the harmonic spectral centroid of the truncated signal s.h.

[0027] memorise each hss(s.h)

[0028] e) calculate the harmonic spectral spread of the signal hss(s) using the following formula: ${{hss}(s)} = \frac{\sum\limits_{nbf}\quad {{hss}\left( {s.h} \right)}}{nbf}$

[0029] where nbf is the number of windows obtained by sliding the window h(t) over the duration D of the signal s(t).

[0030] According to an additional characteristic, a second descriptor called a harmonic spectral deviation (hsd) being used, step d) also includes the calculation of the harmonic spectral deviation of the truncated signal hsd(s(t).h(t)) using the following formula: ${{hsd}\left( {s.h} \right)} = \frac{\sum\limits_{nbh}\quad \left| {{A\left( {{s.h},{harm}} \right)} - {{SE}\left( {{s.h},{harm}} \right)}} \right|}{\sum\limits_{nbh}\quad {A\left( {{s.h},{harm}} \right)}}$

[0031] where SE(s.h,harm) is the local spectral envelope of the truncated signal s.h (with an amplitude at logarithmic scale) around harmonic peak number harm,

[0032] and in that step e) then consists of also calculating the harmonic spectral deviation of the signal hsd(s): ${{hsd}(s)} = \frac{\sum\limits_{nbf}\quad {{hsd}\left( {s.h} \right)}}{nbf}$

[0033] According to one particular embodiment of the invention, the duration of the window h(t) is equal or approximately equal to D and the number of windows nbf is equal to 1.

[0034] The sound signal is preferably a harmonic signal.

[0035] The invention also relates to a process for measurement of the distance “dist” between two harmonic sound signals, characterised in that it consists of using the characterisation of signals like those described above.

[0036] Since the characterisation of sound signals is based on the following descriptors, the logarithmic attack time (lat), the harmonic spectral centroid (hsc), the harmonic spectral deviation (hsd) and the harmonic spectral variation (hsv), the distance “dist” is in the form:

dist={square root}{square root over (x ₁)}(Δlat)² +x ₂(Δhsc)² +x ₃(Δhsd)²+(x ₄ Δhss+x ₅ Δhsv)²

[0037] where x₁, x₂, x₃, x₄, x₅ are predetermined coefficients.

[0038] According to one preferred embodiment, the logarithmic attack time (lat) is calculated on a decimal logarithmic scale and 5<x₁<11, 10⁻⁵<x₂<5×10⁻⁵, 10⁻⁴<x₃<5×10⁻⁴ , 5<x ₄<15 and −30<x₅<−90.

[0039] Other specific features and advantages of the invention will become clearer after reading the following description given as a non-limitative example, and with reference to the attached drawings on which:

[0040]FIG. 1 diagrammatically shows a sound signal s(t) and its time envelope ET(t) as a function of time t;

[0041]FIG. 2 diagrammatically shows a sliding analysis time window h(t);

[0042]FIG. 3 diagrammatically shows harmonic peaks and a spectral envelope ES(f) as a function of the frequency f;

[0043]FIG. 4 diagrammatically illustrates the instantaneous harmonic spectral deviation of a clarinet.

[0044] The sound signal s(t) varying as a function of the time t and a duration D represented in FIG. 1, is analysed according to a sliding time window h(t) shown in FIG. 2, which may for example be a Hamming window.

[0045] The duration D of the signal is usually of the order of a few seconds, for example in the case of sound samples to be located among signals in a database; but it could be much longer.

[0046] According to the invention, a new descriptor representative of the harmonic spectral spread is used to contribute to the description of the timbre of a preferably harmonic sound signal and to enable a more precise calculation of the distance between two sound signals in the same harmonic timbre space.

[0047] The harmonic spectral spread corresponds to a frequency spreading coefficient of the energy of the harmonic part of the signal, about the spectral centroid.

[0048] The calculation of the harmonic spectral spread (hss) includes the following steps carried out on a computer, particularly including one or several memories and a central processing unit comprising at least one microprocessor, a program memory and a working memory:

[0049] a) the signal s(t) with duration D is memorised,

[0050] b) its fundamental period f0 is extracted according to a known process presented above in the description of the state of the art,

[0051] c) harmonics of the signal s(t) truncated according to a time window h(t) such as a Hamming window, for example with a duration N.T0, where T0 is the fundamental period (duration h(t), for example equal to 80 milliseconds where N=8 and T0=10 milliseconds), are calculated starting from the function obtained using a fast Fourier transform program and making the window h(t) slide over the duration D: the position and amplitude of the maxima of this function considered around a multiple of the fundamental frequency f0, determine the frequency and amplitude respectively of the harmonics; these harmonics are memorised;

[0052] d) for each window h(t), the harmonic spectral spread of the truncated signal h ss(s(t).h(t)) is calculated using the following formula: ${{hss}\left( {s.h} \right)} = {\frac{1}{{hsc}\left( {s.h} \right)}\sqrt{\frac{\sum\limits_{nbh}\quad {{A^{2}\left( {{s.h},{harm}} \right)}\left\lbrack {{f\left( {{s.h},{harm}} \right)} - {{hsc}\left( {s.h} \right)}} \right\rbrack}^{2}}{\sum\limits_{nbh}\quad {A^{2}\left( {{s.h},{harm}} \right)}}}}$

[0053] where A(s.h, harm) is the amplitude of harmonic peak number harm of the spectrum of the truncated signal s.h,

[0054] f(s.h, harm) is the frequency of harmonic number harm of the spectrum of the truncated signal s.h,

[0055] nbh is the number of harmonics in the spectrum of the truncated signal s.h,

[0056] hsc(s.h) is the harmonic spectral centroid of the truncated signal s.h calculated according to a state of the art method, for which an example is given later,

[0057] the values hss(s(t).h(t)) thus obtained are memorised.

[0058] e) the harmonic spectral spread of the signal s(t) is calculated as follows: ${{hss}(s)} = \frac{\sum\limits_{nbf}\quad {{hss}\left( {s.h} \right)}}{nbf}$

[0059] where nbf is the number of windows obtained by making the window h(t) slide over the duration D of the signal s(t).

[0060] In the special case of a stationary or quasi stationary signal, the harmonic spectral spread of the signal s(t) is calculated directly over the duration D of the signal. This is equivalent to saying that the duration of the analysis window h(t) is equal or approximately equal to the duration D of the signal and that the number of windows is then equal to 1.

[0061] As soon this new descriptor is available, it can advantageously be combined with the other descriptors lat, hsc, hsd and hsv according to the state of the art, and for example the distance “dist” between two sound signals within the same harmonic timbre space can be calculated using the following formula: ${dist} = \sqrt{{x_{1}\left( {\Delta \quad {lat}} \right)}^{2} + {x_{2}\left( {\Delta \quad {hsc}} \right)}^{2} + {x_{3}\left( {\Delta \quad {hsd}} \right)}^{2} + \left( {{x_{4}\Delta \quad {hss}} + {x_{5}\Delta \quad {hsv}}} \right)^{2}}$

[0062] where Δ is the difference between values of the same descriptor for the two sound signals considered and x₁, x₂, x₃, x₄ and x₅ are predetermined coefficients.

[0063] The logarithmic attack time, lat, is calculated using the formula indicated in the state of the art:

lat(s)=log₁₀(t1−t0)

[0064] For the calculation of the harmonic spectral centroid hsc of the truncated signal, the step d) of the calculation of hss will be completed by the following calculation known to those skilled in the art: ${{hsc}\left( {s.h} \right)} = \frac{\sum\limits_{nbh}\quad {{f\left( {{s.h},{harm}} \right)} \cdot {A\left( {{s.h},{harm}} \right)}}}{\sum\limits_{nbh}\quad {A\left( {{s.h},{harm}} \right)}}$

[0065] In the same way as for the descriptor hss(s) (step e), the following is obtained for the harmonic spectral centroid of the signal s(t): ${{hsc}(s)} = \frac{\sum\limits_{nbf}\quad {{hsc}\left( {s.h} \right)}}{nbf}$

[0066] Step d) in the calculation of hss will advantageously be completed by the following calculation in order to calculate the harmonic spectral deviation hsd of the truncated signal: ${{hsd}\left( {s.h} \right)} = \frac{\sum\limits_{nbh}\quad \left| {{A\left( {{s.h},{harm}} \right)} - {{SE}\left( {{s.h},{harm}} \right)}} \right|}{\sum\limits_{nbh}\quad {A\left( {{s.h},{harm}} \right)}}$

[0067] where SE(s.h,harm) is the local spectral envelope of the truncated signal (with an amplitude at a logarithmic scale) around harmonic peak number harm using a method known to those skilled in the art.

[0068] In the same way as for the descriptor hss(s) (step e), the harmonic spectral deviation of the signal s(t) is given by: ${{hsd}(s)} = \frac{\sum\limits_{nbf}{{hsd}\left( {s \cdot h} \right)}}{nbf}$

[0069] Step d) in the calculation of hss will be completed by the following calculation known to those skilled in the art, in order to calculate the harmonic spectral variation hsv of the truncated signal: ${{hsv}\left( {s \cdot h} \right)} = {1 - \frac{\sum\limits_{nbh}{{A\left( {{{s \cdot h} - 1},{harm}} \right)} \cdot {A\left( {{s \cdot h},{harm}} \right)}}}{\sqrt{\sum\limits_{nbh}{A^{2}\left( {{s \cdot h},{harm}} \right)}} \cdot \sqrt{\sum\limits_{nbh}{A^{2}\left( {{{s \cdot h} - 1},{harm}} \right)}}}}$

[0070] In the same way as for the descriptor hss(s) (step e), the harmonic spectral variation of the signal s(t) is given by: ${{hsv}(s)} = \frac{\sum\limits_{nbf}{{hsv}\left( {s \cdot h} \right)}}{nbf}$

[0071] In particular, the distance was measured by calculating descriptors according to the formulas given above, the logarithmic attack time lat being calculated on a decimal logarithmic scale using coefficients within the following ranges:

5<x ₁<11, 10⁻⁵ <x ₂<5×10⁻⁵, 10⁻⁴ <x ₃<5×10⁻⁴, 5<x ₄<15 and −30<x ₅<−90. 

1. Process for characterisation of the timbre of a sound signal s(t) varying as a function of time for a duration D according to at least one descriptor, characterised in that it consists of defining the said descriptor by the harmonic spectral spread (hss) of the signal.
 2. Process according to claim 1, one of the descriptors being the harmonic spectral centroid (hsc), characterised in that the harmonic spectral spread of the signal is calculated according to the following steps: a) memorise the signal s(t), b) extract its fundamental frequency f0, c) calculate and memorise harmonics of the signal s(t) truncated within a time window h(t) with a duration less than or equal to D, as a function of the frequency using a fast Fourier transform system, making the said time window h(t) slide over the duration D of the signal s(t), d) for each time window h(t), calculate the harmonic spectral spread of the truncated signal hss(s(t).h(t)) using the following formula: ${{hss}\left( {s \cdot h} \right)} = {\frac{1}{{hsc}\left( {s \cdot h} \right)}\sqrt{\frac{\sum\limits_{nbh}{{A^{2}\left( {{s \cdot h},{harm}} \right)}\left\lbrack {{f\left( {{s \cdot h},{harm}} \right)} - {{hsc}\left( {s \cdot h} \right)}} \right\rbrack}^{2}}{\sum\limits_{nbh}{A^{2}\left( {{s \cdot h},{harm}} \right)}}}}$

where A(s.h, harm) is the amplitude of harmonic peak number harm of the spectrum of the truncated signal s.h, f(s.h, harm) is the frequency of harmonic number harm of the spectrum of the truncated signal, nbh is the number of harmonics in the spectrum of the truncated signal s.h, hsc(s.h) is the harmonic spectral centroid of the truncated signal s.h. memorise each hss(s.h) e) calculate the harmonic spectral spread of the signal hss(s) using the following formula: ${{hss}(s)} = \frac{\sum\limits_{nbf}{{hss}\left( {s \cdot h} \right)}}{nbf}$

where nbf is the number of windows obtained by sliding the window h(t) over the duration D of the signal s(t).
 3. Process according to claim 2 and according to which a second descriptor is used, this descriptor being the harmonic spectral deviation (hsd), characterised in that step d) also includes the calculation of the harmonic spectral deviation of the truncated signal hsd(s(t).h(t)) using the following formula: ${{hsd}\left( {s \cdot h} \right)} = \frac{\sum\limits_{nbh}{{{A\left( {{s \cdot h},{harm}} \right)} - {{SE}\left( {{s \cdot h},{harm}} \right)}}}}{\sum\limits_{nbh}{A\left( {{s \cdot h},{harm}} \right)}}$

where SE(s.h,harm) is the local spectral envelope of the truncated signal s.h (with an amplitude at logarithmic scale) around harmonic peak number harm, and in that step e) then consists of also calculating the harmonic spectral deviation of the signal hsd(s): ${{hsd}(s)} = \frac{\sum\limits_{nbf}{{hsd}\left( {s \cdot h} \right)}}{nbf}$


4. Process according to either claim 2 or 3, characterised in that the duration of the window h(t) is equal or approximately equal to D and the number of windows nbf is equal to
 1. 5. Process according to one of the above claims, characterised in that the sound signal is a harmonic signal.
 6. Process for measurement of the distance “dist” between two harmonic sound signals, characterised in that it consists of using the characterisation of signals according to any one of claims 2 to
 4. 7. Process for measuring the distance “dist” according to claim 6, the characterisation of sound signals being based on the following descriptors, the logarithmic attack time (lat), the harmonic spectral centroid (hsc), the harmonic spectral deviation (hsd) and the harmonic spectral variation (hsv), characterised in that the distance “dist” is in the form: ${dist} = \sqrt{{x_{1}\left( {\Delta \quad {lat}} \right)}^{2} + {x_{2}\left( {\Delta \quad {hsc}} \right)}^{2} + {x_{3}\left( {\Delta \quad {hsd}} \right)}^{2} + \left( {{x_{4}\Delta \quad {hss}} + {x_{5}\Delta \quad {hsv}}} \right)^{2}}$

where x₁, x₂, x₃, x₄, x₅ are predetermined coefficients.
 8. Process according to claim 7, characterised in that the logarithmic attack time (lat) is calculated on a decimal logarithmic scale and 5<x₁<11,10⁻⁵<x₂<5×10⁻⁵, 10⁻⁴<x₃<5×10⁻⁴ , 5<x ₄<15 and −30<x₅<−90. 